Conventional FET mixer circuits 20 having a mixing FET 21, and having either of both IF and RF baluns (possibly in the form of a transformer), and exhibiting balanced IF and RF signals are known. The requirement for baluns in these conventional mixer circuits, particularly for the IF balun 22 which operates in a relatively low signal frequency range (typically from about 10 MHz to about 300 Mhz) as a low frequency transformer device, severely limits the ability to fabricate the conventional mixer as a monolithic structure, such as in a monolithic microwave semiconductor integrated circuit (MMIC), given the practical difficulty of providing suitable inductive elements on or within the integrated structure. Even where monolithic fabrication is not a requirement, substantial additional production and material costs are typically incurred when discrete components, especially inductors, are used or added to an otherwise integrated circuit structure.
Mixer-to-mixer operational uniformity may also suffer when discrete components, such as discrete wire-wound inductors, are used in the mixer circuit as a result of the typically higher variability of discrete components relative to the generally lower variability of circuit elements formed as integrated circuit structures. Therefore, additional structures, such as trimming capacitors or the like, may also be required in mixer circuits when discrete components are relied upon.
One type of integrable FET mixer has been marketed by M/A-COM (MD54-001, -003, -004, -005, -006 line of mixers manufactured by M/A-COM, IC Business Unit, 100 Chelmsford St., Lowell, Mass. 01853). This mixer uses a single mixing FET which is not grounded on either end of the channel, and is purported by M/A-COM as having a third order intercept point of about +18 dBm, but this performance has not been verified by the present inventor. The single FET is fed by an Radio-Frequency (RF) balun and the circuit's intrinsically balanced Intermediate-Frequency (IF) output is force coupled, via a diplexer internal to the mixer, to the unbalanced IF port of the mixer. No explicit IF balun is provided, and any distortion reduction that may be provided by the circuit components would necessarily be limited by the single mixing FET implementation of the circuit.
Therefore while there have been attempts to provide a fully integrable mixer having a large dynamic range, excellent linearity, and low distortion, the prior-art mixers by and large fail to satisfy the need for fully integrable mixers that provide the degree of linearity with distortion suppression or complete distortion cancellation desired, and which also eliminate the need for IF and RF baluns. Such high-performance, fully integrated implementations are highly desirable for compact communication products, including cost-efficient wireless communication products for base station of remote communication capability. Such products include cellular and PCS wireless telephones. For these and other reasons there continues to be a need to provide high-performance mixers that eliminate IF and RF baluns and are capable of full integration on a monolithic substrate.